Precision fit for bearings and the like



Jan. 25, 1955 L. P. MIGNY 2,700,581

PRECISION FIT FOR BEARINGS AND THE LIKE Filed March 3, ,1954 3 Sheets-Sheet l FiG.2

I N VENTOR Louis P. Mig ny BY W ATTORNEY Jan. 25, 1955 L. P. MIGNY PRECISION FIT FOR BEARINGS AND THE LIKE 3 Sheets-Sheet 2 Filed March 3, 1954 Fia4- Fie. 5

INVENTOR V Louis'P. Migng ATTORNEY Jan. 25 1955 L. P. MIGNY 2,700,581

PRECISION FIT FOR BEARINGS AND .THE LIKE Filed March 5, 1954 3 Sheets-Sheet 3 Fie.7

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Lo uisRMigny ATTORNEY 2,700,581 Patented Jan. 25, 1955 PRECISION FIT FOR BEARINGS AND THE LIKE Louis P. Migny, Paris, France, assignor to lAluminium Frangais, Paris, France, a corporation of France Application March 3, 1954, Serial No. 413,865

Claims priority, application France March 13, 1953 9 Claims. (Cl. 308-238) The present invention relates to precision fits of parts formed of materials which do not have the same coetticient of expansion.

it is generally impossible to produce high precision fits of cylindrical steel elements in metallic parts of, for example, aluminum and magnesium alloys, when the temperature varies some tens of degrees. On the one hand, the coetiicient of expansion of aluminum alloys (20 to Z3 lU-), or of magnesium alloys (26 (l0- are quite ditferent from those of steels (10 to 14X 10- on the other hand, the annual variation of the ambient temperature alone approaches 50, while a large number of machines are additionally designed to be subjected to variations of temperatures produced as a result of wide spreads of latitude and altitude.

The resultant dimensional variations produce either excessive play at the highest temperatures or, else, unpermissible tightness at the lowest temperatures, which is incompatible with the normal operation of the parts or, even, their durability.

The present invention has for its object to overcome this drawback. Essentially, it consists in lining the bore of the part, or housing, in which the cylindrical steel part is mounted, with a solid material which simultaneously possesses, relatively to the material of the housing, a considerably larger coefiicient of expansion, and a considerably smaller modulus of elasticity; the applied solid material constitutes a cylindrical ring which is interposed between said bore and said steel part.

Applicant has discovered that it is possible to carry out his invention in two different ways:

(a) The interposed ring is connected to the housing either by cementing thereto or, simply, by its own elasticity. It is thereupon possible, if the ratio of the internal and external diameters of the ring possesses a well defined value relatively to the same ratio of the diameters of the part of the housing where the fitting takes place, to obtain the result that the apparent coetficient of expansion of the interior diameter of the ring be the same as that of the steel part, which eliminates all variation in the lit by reason of temperature (variations). If the fit be tight, then, the pressure exerted on the steel part remains substantially constant when the temperature varies; if the fit permits sliding, it remains a sliding fit at all temperatures, without appreciable variation in play.

(b) The interposed ring is fastened to the inner steel part, either by being cemented thereto, or merely by its own elasticity. It is thereupon possible, if the ratio between the internal and external diameters possesses a value which is properly related to the same ratio of the diameters of the inner part to which it is fastened, to obtain the result that the apparent coefiicient of the external diameter of the said part be the same as that of the bore of the housing in which the assembly is mounted, which eliminates all variation of fit in the housing by reason of temperature (variations) under the same conditions as previously described.

Applicant has analyzed mathematically the conditions for mechanical equilibrium in the case of three concentric parts where the interposed-intermediate-ring is continuous (i. e. does not have any break), and where tensions parallel to the axis of the ring are neglected. In the following analysis, symbols pertaining to the central steel part, will be designated by the suffix 0, intermediate ring will be designated without a sufiix, the external housing will be designated by the suflix 1.

Let Do D Di represent the respective external diameters; a'n a d1 represent the respective internal diameters; a0 a a represent the respective linear thermal coeiiicients of expansion; E0 E E1 represent the respective elasticity moduli (or Young moduli); mo m m1 represent the respective Poisson ratios; Q0 Q Q1 represent the respective dimensional moduli whereby Q equals Applicant has discovered that the condition defined under paragraph (a) above is satisfied by the following equation:

and that the condition defined under paragraph (b) above is satisfied by the equation (Q-l) han (ll- 1) (g wrmn +Q+m) it is advantageous that the material of which the intermediate ring is constituted have a coeflicient of expansion ranging between SOXlO- and 10 a modulus of elasticity less than 700 kg./mm.'-, and an elastic limit on compression greater than 2 kg./mm.

Under these conditions, the ratio between the external and internal diameters of the intermediate ring preferably varies between 1.02 and 1.30, according to circumstances.

With the foregoing in mind, applicant has found that the intermediate ring can be suitably formed of ebonite or hard rubber, and of a large number of plastics, polyarnides or nylons being specially adapted for this purpose. The selection of a particular material is governed by the other conditions to be satisfied, for example, be havior in the presence of lubricating oils, hydrocarbons, or aqueous products, temperature ranges, etc.

A particularly interesting application of the present invention pertains to the mounting of ball bearings in housings of motors or of machines, and there will be given below an example of such an application. However, it is evident that the method can be used in every case where it is desired to compensate a harmful ex pansion in order to obtain a more accurate fit, as for example, in the case of plungers of hydraulic pumps or related apparatus, pistons of fuel injection pumps for heat engines, etc.

The apparent coefiicient of expansion of the internal bore of the ring can be reduced to zero or, at least, very close to zero, thereby enabling application of the present invention to metrology (precise measurements).

In carrying out the method of the invention, the intermediate ring can be cast or injected in place in the housing, or else produced separately with suitable (end) side dimensions. In the latter case, it can be provided with a side dimension slightly larger than the recess in which it is to fit and be press-fitted therein. It can also be cemented in place, or else forced in by hammering or by any other method resulting in auto-frettage (selfhooping).

It is also possible to separately manufacture these nonbinding rings in either simple or composite form, the latter comprising a thin internal steel ring and, when required, a thin external ring formed of the same material as that of the housing in which the non-binding ring is to be mounted.

In the annexed drawings, there are shown by way of example several embodiments of the invention wherein:

Figures 1 and 2 represent exploded views-in cross section-of two different embodiments of the invention as applied to the mounting of ball bearings in housings of light alloys.

Figure 3 represents a still different embodiment;

Figure 4 represents a section along a radial plane through the circumference of a special construction of an anti-binding (non-seizing) ring of the present invention;

Figure represents a section, likewise along a radial plane, of the same ring fitted in a recess in a housing.

Figure 6 is a section along a radial plane of a ball bearing, whose outer ring is constructed as a non-seiz ing ring in accordance with the special form of the present invention illustrated in Figs. 4 and 5.

Figure 7 is a graph illustrative of the present inventron.

In the various figures, the same or equivalent parts are designated by the same reference numerals.

By reference to Figure I, it will be observed that the ball-bearing assembly designated as a whole by numeral 1 is here provided with a ring' or lining 2 of plastic material which is positioned and retained in a circumferential recess on the exterior of the outer race 10. When the ball-bearing mounting is placed in position in the bore 5 of the housing 4, and the parts are suitably fitted together in accordance with the teachings set out above, the fit will be maintained throughout the operation of the device despite wide variations in temperature. As will be observed, the housing. 4, which has a; greater coefiicient of expansion than the material of the ball bearing; assembly, is positioned externally of the latter. The same order of placementof parts holds true in the case of the embodiment illustrated in Figure 2. Here, however; the plastic. liner or ring 2 is attached-in any suitable mannerto the internal surface of the bore 5 in the housing 4.

In the. embodiment illustrated in Figure 3, the steel shaft. 6. fits within a bore in the light alloy housing 4 through the intermediary of the plastic ring 2. Here again, the metal of the housing has a greater coefiicient of, expansion than the steel shaft 6, and is external to the latter. As in the case of the embodiments of Figures land 2', the plastic ring, when suitably dimensioned, in accordance with applicants teachings herein, functions to maintain the desired fit between shaft and hous in'g despite wide temperature variations that may occur during operation of the assembly.

As has been mentioned previously one successful, practical embodiment of the anti-binding ring or lining inaccordance with the present invention has a composite structure. In general, it consists of an inner ring of steel provided with lateral shoulders, between which is fitted another ring constituted of a plastic material having a polyamide base. In the case of such composite rings,- consideration must be given to the following:

1. Polyamide base plastics progressively lose their elastic resistance properties with rise in temperature and are subject to a solid flow phenomenon, with possible resultant variation of their (relative) thickness; such flow only becomes appreciable at temperatures above 100 C., a temperature which is generally not attained in normal operation in the case of the majority of mechanisms, but may be accidently exceeded in certain cases.

2; The total thickness of the composite ring must evidently be provided for in the housing, in addition to the radius of the part to be fitted, and this leads to an excessive bore size, which may be harmful.

Applicant has solved the difficulties above enumerated by adopting the special construction of a composite ringillustrated in Figures 4-6. erence will also be had to Figure 7 which represents a graph showing the relationship between temperature and variations in fit; the latter were deduced from practical tests which compared a ball bearing of ordinary construction, and a bearing of the same construction but modified according to the present invention, both of these bearings having been pressfitted into an aluminum alloy housing. The non-seizing ring of the present invention used in these tests was constructed as shown in Figure 4 and comprised a steel ring 11, provided on its circumference with a recess 12 into which is cast or- (hot) injected a lining material 2, for example, a pol'yamide-base plastic. This recess is bound at either side by shoulders 13, and its bottom is formed of' fine circumferential grooves 14' which, in the righthand portion" thereof (looking at Figure 4'), possess each on their left, a plane annular support surface 15; on the otherhand, the lefthand portion of the same recess 12 present on theirright analogous supporting surfaces 16. The lining material'2 is defined at its exterior by In this connection, refa cylindrical surface of diameter D; slightly larger than the external diameter D2 of the lateral shoulder on the steel ring. The total thickness 2 to be given to the said material is a function of the coefiicient of expansion a of the liner material, of an of the steel, and of d1 of the housing material. The number and the dimensions of the grooves 14 are determined by the condition that, the total area of the supporting surfaces 15 (or of the supporting surfaces 16, which is equal thereto) be distinctly larger than the annular cross section of the liner material at its central portion, which is designated on Figure 4 by the thickness e1.

When the said anti-seizing ring is mounted (as shown in Figure 5) in the light-alloy or ultra-light alloy housing 4 by either a press or sliding fit; it is necessary that the internal diameter Di of the bore in the housing, and the external diameter D2 of the lateral shoulders 13 on the steel ring 11, differ by a sufiicient amount, so that. at the lowest operating. temperature of the device, and taking into account the difference (a1ao) in the expansion coefiicients, the two parts do not come into contact. However, this difference in the diameters is a very small quantity and, as can be seen from Figures 4-6, the material of the liner is entirely enclosed by the metal, except at each side where there is a circular (exposed) area the width i of which is generally but a fraction of a millimeter. The width i will, in practice, by reason of rnanufacturing tolerances, generally range between 0.0005 and 0.0015D. Should the temperature rise sufficiently so that there is. a risk that solid flow may take place, then, it would be necessary; in order that the material actually flow through. such small openings, to have present pressures infinitely larger than those than would be actually present in practice; the result is, that the material is retained in the space provided therefor, which solves the first of the difficulties indicated above under heading 1.

As regards the second difficulty above mentioned, un: der heading 2, it should be observed that inasmuch as the liner material is hot cast or hot injected, cooling thereof produces a contraction which is appreciably larger in the case of the liner material than in case of the steel, since the material has deliberately been se-' lected on the basis of its very high coefficient of expansion. It follows that, at the operating temperatures, the said material is subjected to an elastic expansion both in a circumferential direction since a hooping effect (frettage) takes place here as well as in a direction parallel to the axis, by reason of the attachment of the material to the supporting surfaces 15 and 16.

This double expansion, which varies with the temperature, produces a proportional deformation in a radial direction which is added to the linear expansion of the material in this direction.

Summarizing, it can be stated that if, by reason of the desired (selected) wide spread between the coefficients of expansion a and as, we neglect the effect of Poissons ratio, then, virtually the entire cubic' coefficient of expansion 311 takes place in the radial direction. This is because, in the two other dimensions (directions), the expansion of the material is' limited to that of the small value (amount) an of the steel. The" considerable increase in the radial expansion makes it possible to' use a very thin layer of liner material.

Actually, it has been found that in the case of a polyamide plastic material, it is sufficient that the value of e range between 0.012 and 0.018D for mountings in aluminum alloy housings, and between 0.016 and 0.025D in the case of magnesium alloy housings. The small size of this thickness makes it possible to reduce the overall bulk of the antibinding ring. But, in addition, it is sufficiently small so that when the outer race of a ball bearing (Figure 6), roller bearing, or needle bearing, is formed-into an anti-binding ring, the plastic material can be entirely located within the normal thickness of the steel race (ring), so that there is no increase in the (dimension of the) standardized external diameter D and hence; there is no need for increasing the size of the bore (in the housing).

The graphs shown in Figure 7 have been drawn from data obtained by measuring the deformation with temperature of rings of an aluminum alloy, designated Alloy Y, containing about 4% copper, 2%" nickel and 1.5% magnesium, into which rings there wererespectively press-fitted (l) a commercial roller bearing (Graph A),

and (2) the same type of bearing the exterior of which was, however, converted into an anti-binding ring in accordance with the present invention by being provided with a graphitized Rilsan lining of a thickness e=0.014D (Graph B). The external diameter of each of the bearings was 85 mm., and there were 7 circular grooves on each side on the bottom of the recess in the case of the construction according to the present invention, each groove being 0.2 mm. deep.

As will be readily apparent from an inspection of Figure 7, the bearing provided with an intermediate liner in accordance with the present invention substantially maintained the same degree of fit with reference to its housing throughout wide variations of temperature (Graph B). In contrast thereto, the ordinary bearing suffered considerable variation in the quality of its fit (Graph A).

Graph B shows that while the compensation for expansion is not absolutely complete in this particular case, variations of fit in the range between and +80 C. are not incompatible with the permissible tolerances of fit of parts according to standard IT-Z. The fit of such parts is therefore very accurate despite temperature variations.

The Rilsan" liner material referred to above is formed of polyamide derivative of ricinoleic acid. Nylon may also be employed.

I claim:

1. An assembly of machine parts comprising in combination: a cylindrical element; a housing constituted of a material having a higher thermal coefficient of expansion than the material of said element; a bore in said housing, said cylindrical element being disposed in said bore; a liner comprising a plastic material having a considerably higher thermal coefiicient of expansion and a much lower modulus of elasticity than the material of the housing, said liner being interposed between the housing and said element whereby, when the parts in assembled relationship form a precision-fit assembly, the initial precision fit will be maintained substantially between said parts over a wide range of operating temperatures.

2. An assembly according to claim 1 wherein the liner comprises a plastic material having a thermal coefiicient of expansion ranging between 5O l0 and 150 l0 a modulus of elasticity not in excess of 700 kgs./mm. and an elastic limit on compression greater than 2 kg./mm.

3. An assembly according to claim 1, wherein the liner is in the form of a ring, the ratio of whose external to internal diameter ranging between 1.02 and 1.3.

4. An assembly according to claim 1 wherein the cylindrical element is formed of steel, and the housing which surrounds it is formed of an alloy of a metal selected from the group consisting of aluminum and magnesium.

5. An assembly according to claim 1 wherein the liner comprises a supporting steel ring provided with an external circumferential recess, and the plastic material is disposed in said recess.

6. An assembly according to claim 5, wherein the recess is provided at each lateral end thereof with an upstanding shoulder, and the bottom of the recess is provided with fine circumferential grooves; and wherein at least one side of the groove is constituted of a flat annular plane, the said sides being symmetrically disposed with reference to the shoulders, and serving as anchoring surfaces for the plastic material.

7. An assembly according to claim 5, wherein the recess is provided at each lateral end thereof with an upstanding shoulder, and the bottom of the recess is provided with fine circumferential grooves; and wherein at least one side of the groove is constituted of a fiat annular plane, the said sides being symmetrically disposed with reference to the shoulders, and serving as anchoring surfaces for the plastic material; said assembly being further characterized in that the plastic material has a polyamide base and has a thickness ranging between 0.012 and 0.018 of its external diameter, and the housing is formed of an aluminum alloy.

8. An assembly according to claim 5 wherein the recess is provided at each lateral end thereof with an upstanding shoulder, and the bottom of the recess is provided with fine circumferential grooves; and wherein at least one side of the groove is constituted of a fiat annular place, the said sides being symmetrically disposed with reference to the shoulders, and serving as anchoring surfaces for the plastic material; said assembly being further characterized in that the plastic material has a polyamide base and has a thickness ranging between 0.016 and 0.025 of its external diameter and the housing is formed of a magnesium alloy.

9. An assembly according to claim 4, wherein the cylindrical element comprises an anti-friction bearing.

References Cited in the file of this patent UNITED STATES PATENTS 

